In commercial container filling or packaging operations, the containers typically are moved by a conveying system at very high rates of speed. In current bottling operations, copious amounts of aqueous dilute lubricant solutions (usually based on ethoxylated amines or fatty acid amines) are typically applied to the conveyor or containers using spray or pumping equipment. These lubricant solutions permit high-speed operation (up to 1000 containers per minute or more) of the conveyor and limit marring of the containers or labels, but also have some disadvantages. For example, aqueous conveyor lubricants based on fatty amines typically contain ingredients that can react with spilled carbonated beverages or other food or liquid components to form solid deposits. Formation of such deposits on a conveyor can change the lubricity of the conveyor and require shutdown to permit cleanup. Some aqueous conveyor lubricants are incompatible with thermoplastic beverage containers made of polyethylene terepihthalate (PET) and other plastics, and can cause stress cracking (crazing and cracking that occurs when the plastic polymer is under tension) in carbonated beverage filled plastic containers. Dilute aqueous lubricants typically require use of large amounts of water on the conveying line, which must then be disposed of or recycled, and which causes an unduly wet environment near the conveyor line. Moreover, some aqueous lubricants can promote the growth of microbes.
Thermoplastic materials have been used for many years for the formation of thermoplastic materials in the form of film, sheet, thermoformed and blow molded container materials. Such materials include polyethylene, polypropylene, polyvinylchloride, polycarbonate, polystyrene, nylon, acrylic, polyester polyethylene terephthalate, polyethylene naphthalate or co-polymers of these materials or alloys or blends thereof and other thermoplastic materials. Such materials have been developed for inexpensive packaging purposes. Thermoplastic materials are manufactured and formulated such that they can be used in thermoforming processes. Such thermal processing is used to form film, sheet, shapes or decorative or mechanical structures comprising the thermoplastic material. In such processes, the thermoplastic is heated to above the glass transition temperature (Tg) or above the melting point (Tm) and shaped into a desirable profile by a shaping die. After the shape is achieved, the material is cooled to retain the shape. The cooling of such materials after shaping can often lock-in stresses from the thermal processing. Filling such a container with carbonated beverage can place large amounts of stress in the bottle structure. Most thermoplastic materials when stressed react undesirably to the stress and often relieve the stress through cracking. Such cracking often starts at a flaw in the thermoplastic and creeps through the thermoplastic until the stress is relieved to some degree.
Such stress cracking can be promoted by stress cracking promoter materials. Thermoplastics that are highly susceptible to stress cracking include polyethylene terephthalate, polystyrene, polycarbonate and other thermoplastics well known to the skilled materials scientist. The mechanism of stress crack promotion, initiation and propagation has been discussed and investigated but not clearly delineated. Stress cracking can be explained by discussing interactions between stress cracking promoters and the polymeric chains that make up the thermoplastic material. The stress cracking promoters are believed to cause one or more chain to move relative to another chain, often initiated at a flaw in the plastic, resulting in cracking. Other theories include a consideration of the chemical decomposition of the thermoplastic material or (e.g.) a base catalyzed hydrolysis of the polyester bond resulting in weakened areas in the thermoplastic resulting in associated cracking. Lastly, the thermoplastic materials are believed to absorb more hydrophobic materials that soften the thermoplastic and, by reducing the strength of the thermoplastic, can promote the growth and propagation of stress cracking.
Regardless of the theory of the creation and propagation of stress cracks, thermoplastics manufacturers are well aware of stress cracking and have sought to develop thermoplastic materials that are more resistant to stress cracking. Stress cracking can be reduced by sulfonating the bulk thermoplastic after formation into a final article. Further, the manufacture of containers in two, three, four or other multilayer laminate structures is also believed to be helpful in reducing stress cracking. However, we have found that even the improved polymer materials can be susceptible to stress cracking. Further, certain commonly used container structures including polystyrene materials, polycarbonate materials, polyethylene terephthalate materials tend to be extremely sensitive to stress cracking promoters particularly when pressurized or used at high altitudes and can during manufacture, use or storage quickly acquire a degree of stress cracking that is highly undesirable.
One technology involving significant and expensive stress cracking involves the manufacture of polyethylene terephthalate (PET) beverage containers. Such beverage containers are commonly made in the form of a 20 oz, one, two or three liter container for carbonated beverages. Alternatively, a petaloid design can be formed into the polyester to establish a stable base portion for the bottle. In both formats, the polyester beverage container can have significant stress formed in the shaped bottom portion of the bottle. The stresses in the pentaloid structure tend to be greater because of the larger amorphous region and more complex profile of the container base.
Polyester beverage containers are made in a two step process. Melt thermoplastic is formed into a preform. Such preforms are relatively small (compared to the finished bottle) comprising the threaded closure portion and a “test tube” like shape that is blow molded into a final bottle conformation. In manufacturing the beverage containers, the preform is inserted into a blow molding apparatus that heats the preform and, under pressure, inflates the softened preform forcing the preform into a mold resulting in the final shape. The finished beverage containers are shipped to a filling location. The containers are filled with carbonated beverage in a filling apparatus that involves a moving conveyor surface that tnansports the container during filling. The conveyor structure comprises a filling station, a capping station and ends at a packing station. While on the conveyor, the containers are exposed to an environment that contains residual cleaners and conveyor lubricants containing organic and inorganic stress cracking components that can interact with the polyester thermoplastic of the container. Stress cracking can appear as fine cracking that typically forms axially around the center of the bottle. The appearance of any stress cracking is undesirable. Should beverage containers stress crack, the pressure of the carbonated beverage can often cause the beverage container to explode and spill the beverage contents in the processing plant, transportation unit, warehouse or retail outlet. Such spillage poses health problems, sanitation problems, maintenance problems and is highly undesirable to manufacturers and retail merchants.
Initially such conveyor systems were lubricated using dilute aqueous lubricant materials. Typical early conveyor lubricants comprise substantially soluble sodium salt of the fatty acid or sodium salt of linear alkane sulfonate which acted to both lubricate and at least to some degree, clean the conveyor surfaces. Representative examples of such lubricants are found in Stanton et al., U.S. Pat. No. 4,274,973 and Stanton, U.S. Pat. No. 4,604,220. When conventional aqueous conveyor lubricant compositions were applied to conveyors for polyester beverage containers, the lubricants were found to be significant stress crack promoting materials. No clear understanding of the nature of stress crack promotion is known, however, the lubricant compositions containing basic materials (caustic and amine compounds) appear to be stress crack promoters. Such materials include aqueous soluble sodium salts, aqueous soluble amine compounds, and other weak to strong aqueous soluble bases have been identified as stress crack promoters. Other stress cracking promoters include solvents, phenols, strong acids, alcohols, low molecular weight alcohol ethoxylates, glycols and other similar materials.
A series of allegedly stress crack inhibiting substantially soluble aqueous lubricants were introduced including Rossio et al., U.S. Pat. Nos. 4,929,375 and 5,073,280; and Wieder et al., U.S. Pat. No. 5,009,801. These patents assert that certain substituted aromatic compounds, certain couplers and saponifying agents and certain amine compounds can inhibit stress cracking in appropriately formulated materials. Other patents, including Person Hei et al., U.S. Pat. Nos. 5,863,874 and 5,723,418; Besse et al., U.S. Pat. No. 5,863,871; Gutzmann al., U.S. Pat. Nos. 5,559,087 and 5,352,376; Liu et al., U.S. Pat. No. 5,244,589; Schmitt et al., U.S. Pat. No. 5,182,035; Gutzmann et al., U.S. Pat. No. 5,174,914; teach conveyor lubricants that provide adequate lubrication, cleaning and inhibit stress cracking.
In many applications, known improved stress cracking resistant thermoplastic materials cannot be used for reasons of cost or poor processability properties. A substantial need exists for improved methods of preventing stress cracking in shaped thermoplastic materials in any environment. Important harsh environments include a stress crack promoter.
Containers are receptacles in which materials are or will be held or carried. Containers are commonly used in the food or beverage industry to hold food or beverages. Often lubricants are used in conveying systems for containers, to ensure the appropriate movement of containers on the conveyor.
In the commercial distribution of many products, including most beverages, the products are packaged in containers of varying sizes. The containers can be made of paper, metal or plastic, in the form of cartons, cans, bottles, Tetra Pak™ packages, waxed carton packs, and other forms of containers. In most packaging operations, the containers are moved along conveying systems, usually in an upright position, with the opening of the container facing vertically up or down. The containers are moved from station to station, where various operations, such as filling, capping, labeling, sealing, and the like, are performed. Containers, in addition to their many possible formats and constructions, may comprise many different types of materials, such as metals, glasses, ceramics, papers, treated papers, waxed papers, composites, layered structures, and polymeric materials.
Lubricating solutions are often used on conveying systems during the filling of containers with, for example, beverages. There are a number of different requirements that are desirable for such lubricants. For example, the lubricant should provide an acceptable level of lubricity for the system. It is also desirable that the lubricant have a viscosity which allows it to be applied by conventional pumping and/or application apparatus, such as by spraying, roll coating, wet bed coating, and the like, commonly used in the industry.
In the beverage industry, the lubricant must be compatible with the beverage so that it does not form solid deposits when it accidentally contacts spilled beverages on the conveyor system. This is important since the formation of deposits on the conveyor system may change the lubricity of the system and could require shutdown of the equipment to facilitate cleaning.
The lubricant must be such that it can be cleaned easily. The container and/or the conveyor system may need to be cleaned. Since water is often in the cleaning solution, ideally the lubricant has some water-soluble properties.
Currently, containers, including polyethylene terephthalate (PET) bottles, and conveying systems for containers are often contacted with a volume of a dilute aqueous lubricant to provide lubricity to the container so that it can more easily travel down the conveyor system. Many currently used aqueous-based lubricants are disadvantageous because they are incompatible with many beverage containers, such as PET and other polyalkylene terephthalate containers, and may promote stress cracking of the PET bottles.
Furthermore, aqueous based lubricants are in general often disadvantageous because of the large amounts of water used, the need to use a wet work environment, the increased microbial growth associated with such water-based systems, and their high coefficient of friction. Moreover, most aqueous-based lubricants are incompatible with beverages.
Flooding a conveyor surface with a substantial proportion of aqueous lubricant typically occurs on food container filling or beverage bottling lines. Sufficient lubricant is used such that the lubricant is not retained entirely by the surface of the conveyor but tends to flow from the surface of the container, drip onto a conveyor support members and the surrounding environmental area around the conveyors. Further, sufficient amounts of lubricant are applied to the conveyor and other mechanisms of the plant under such conditions that a substantial foam layer of lubricant can form on the surface of the conveyor. As much as one inch (about 2.5 cm or more) thick of lubricant foam can contact a substantial portion of the base of a food container such as polyethylene terephthalate beverage bottle. We have found that current methods of lubricating such containers are wasteful of the lubricant material since a substantial proportion of the materials is lost as it leaves the container surface. Further, substantial proportions of the lubricant remain on the container and are carried from the conveyor as the food packaging or beverage-bottling operations are continued. A substantial need exists for approved methods that waste little or no lubricant during packaging or bottling operations.
The tendency of polyester (PET) beverage containers to crack or craze is promoted by the presence of a number of common lubricating materials in contact with a substantial proportion of the surface of a polyester beverage container under pressure. The stress arises during manufacture of the polyester bottle from a preform. The stress is locked into the beverage container during manufacture and is often relieved as the lubricant materials contact the bottle. Lubricant materials appear to promote movement of the polyester molecules with respect to each other, relieving stress and leading to the creation of stress cracking. We have found that the degree of stress cracking is attributable, at least in part, to the amount of surface area of the bottle contacted by the lubricant. We have found in our experimentation that limiting the amount of surface area of the bottle that comes in contact with the lubricant can substantially improve the degree of stress cracking that occurs in the bottle material. Clearly, a substantial need exists to develop lubricating methods that result in the minimum amount of lubricant contact with the surface of the food container.